1. Field of the Invention
The present invention relates to a liquid crystal electro-optical device such as a display device, a shutter array for printers, a shutter for camera or an optical shutter for window, using a ferroelectric liquid crystal having a negative dielectric anisotropy. More particularly, the present invention relates to an improvement in AC-field stabilization effect using phenylpyridazine liquid crystal materials.
2. Description of the Related Art
After Clark and Lagaval proposed a surface-stabilizing method in 1980, intensive investigations have been made for the purpose of utilizing a ferroelectric liquid crystal element as a display element or shutter array.
However, when a ferroelectric liquid crystal phase is formed in an oriented liquid crystal cell, the layers of the smectic phase are bent to form a so-called chevron structure as shown in FIG. 1A or the layers slant and the apparent cone angle between the bi-stable states is narrow as shown in FIG. 1B. Thus an angle of about 45.degree. indicating an ideal orientation, the switching between the bi-stable states, as shown in FIG. 2 cannot be obtained. Further, when the interaction between the substrate surface and the spontaneous polarization of the ferroelectric liquid crystal is strong, no uniform orientation but a twist orientation is obtained.
A problem that the optical ON-OFF density ratio (contrast) is low for the above-described reasons has thus not been solved. Known processes for increasing the contrast include a process wherein an oblique deposited silicon oxide film is used as the oriented film to realize the bi-stable orientation state which is nearly the ideal orientation state and a process wherein a ferroelectric liquid crystal having a negative dielectric anisotropy is used in order to utilize the AC stabilized effect.
However, the process wherein an oblique deposited silicon oxide film is used as the oriented film is unsuitable for the mass-production and it necessitates a high cost, since a large vapor deposition apparatus is necessitated for forming a uniform film on a large substrate and a long time is taken for the film formation. A process wherein a ferroelectric liquid crystal having a negative dielectric anisotropy is used for utilizing the AC stabilized effect is disclosed in Japanese Patent Laid-Open Nos. 176097/1985, 192020/1988, 201629/1988, 210825/1988, etc. However, a high voltage and a high frequency are necessitated for obtaining a high AC stabilized effect and, therefore, a driver IC used in an ordinary driver circuit of a liquid crystal display element cannot be used or a complicated driver circuit is necessitated.
Generally a ferroelectric liquid crystal having a negative dielectric anisotropy of a high absolute value tends to bring about a strong intermolecular interaction and a high response speed. Accordingly, the width of the voltage pulse necessary for the inversion of the liquid crystal molecule is large and the width of the voltage pulse producing the AC stabilized effect is small. An ordinary driving method comprises, therefore, superimposing high-frequency AC waveforms generated from a signal electrode.
The AC stabilized effect-producing voltage significantly varies also depending on the orientation state of the ferroelectric liquid crystal element. Namely, the stabilized effect-producing voltage is low when the orientation is in the uniform, bi-stable states, but the voltage is quite high when the orientation is twisted. To solve this problem, it has been demanded to produce uniform, bi-stable orientation states by suitably combining a ferroelectric liquid crystal material with an oriented film.